1. It is impossible, practically speaking, to measure units of "opportunity" which we can then distribute equally.
2. People vary in what they most want. One issue we all face in the workplace is what ratio of income and wealth, on the one hand, and discretionary time, on the other, we prefer. Requiring everyone to choose the same "outcome" in this regard is neither possible nor desirable.
3. Everytime it has been tried, this goal and standard has been incinerated in the flames of discontent. The New Testament testifies to one of the most spectacular of these test cases because it was motivated by religious generosity. In "Acts of the Apostles" we read that the very first Christians possessed everything "in common" and distributed everything according to "need. It did not take long for this way of doing things to prompt heated division along ethnic and cultural lines. (Acts 2, 4 and 6)
4. The problem of "freeloading" erupts everywhere and always. Among the earliest Christians, this difficulty became so great that the Apostle Paul," or someone writing in his name, declared that "Anyone unwilling to work should not eat." (II Thessalonians 3:10)
5. "Equality of opportunity" denies the principle, which I think that even those who negotiate behind the "veil of ignorance" in John Rawls' "hypothetical original position" would affirm, that within broad boundaries which have to be drawn on a case-by-case basis by those who are most involved, those who achieve and contribute more should also receive more.
6. As John Rawls "difference principle" addresses, sometimes those who end up with the least in unequal allocations receive more than if the outcomes had been distributed equally. My reservation about the "difference principle" as Rawls formulates, it is that I think that his hypothetical negotiators would contend for some proportionality between how much more the best off get and how much more the worst off get.
7. "Equality of opportunity" denies citizens the opportunity of freely contributing to the income and wealth of those "performances" they enjoy. These can range from athletes to artists and scientists.
Although I won't spell it out, I think it possible to demonstrate that each one of these can be slightly modified so as to count against "equality of opportunity" with plausibility too. If this is so, and I think it is, we need a different goal and standard.
I favor making it "equity and utility of opportunity." "Equitable" means "fare" and not "equality." What is "fair" cannot be determined in advance. It has to be worked out on a case-by-case basis by those who are most involved, using whatever mental and material resources they have.
This means changing John Rawls' "hypothetical original position" to a "hypothetical continuing position."